It is well known in the art of styrene manufacture to react ethylbenzene (EB) in the presence of steam over a dehydrogenation catalyst, such as iron oxide under dehydrogenation reaction conditions, in order to strip hydrogen from the ethyl group on the benzene ring to form styrene. It is also well known that the dehydrogenation of ethylbenzene requires large amounts of energy, for example, in the form of steam.
Alternative methods for reducing energy consumption (i.e., steam) in processes for producing styrene via dehydrogenation of ethylbenzene have been previously described.
U.S. Pat. No. 4,628,136 to Sardina discloses a dehydrogenation process for producing styrene from ethylbenzene in the presence of steam by recovering heat of condensation normally lost during separation of the various components and using the heat to vaporize an aqueous feed mixture of ethylbenzene and water. Sardina teaches that this obviates the need to use steam to vaporize the liquid ethylbenzene feed.
U.S. Pat. No. 4,695,664 to Whittle discloses a means for recovering waste heat from a low temperature process stream with a vaporizable heat sink liquid and two immiscible liquids that form a low boiling azeotrope. The heat sink liquid is brought into indirect heat exchange with the low temperature process stream, whereby the heat sink liquid is able to recover heat from the process stream.
Various methods have been proposed that allow use of azeotropic heat recovery while operating at the minimum ratio of reaction steam to ethylbenzene, as determined by catalyst stability (i.e., resistance to coking). Such methods include use of direct heating as described in U.S. Pat. Nos. 8,193,404 and 8,084,660 to Welch et al., which discloses among other things methods for increasing the efficiency of a dehydrogenation unit by use of at least one direct heating unit.
Method of providing heat for chemical conversion and a process and system employing the method for the production of olefin to U.S. Pat. No. 8,163,971 to Wilcox et al. addresses the problem of supplying heat to the system at an overall steam/oil weight ratio of 1.0 or lower. Generally, these ratios would require steam temperature at the outlet of the steam superheater to be increased to 950° C., or even higher. At such high temperatures, the use of special and costly metallurgy is required.
U.S. Pat. No. 7,922,980 to Oleksy et al. discloses methods for recovering the heat of condensation from overhead vapor produced during ethylbenzene-to-styrene operations. In this regard, the '980 patent uses the overhead of an EB/SM splitter column to vaporize an azeotropic mixture of ethylbenzene and water.
Other methods that could be employed to enable the use of azeotropic heat recovery while operating at the minimum ratio of reaction steam to ethylbenzene involve passing the reactor feed mixture through the convection section of a fired heater, as practiced by The Dow Chemical Company as described in U.S. Pat. No. 4,769,506 to Kosters.
Use of a split reheater arrangement as disclosed in published International Application No.: PCT/US2012/053100, Pub. No. W0/2014/035398, makes it possible to reduce the heating steam to ethylbenzene ratio required for interstage reheat to as low as 0.34 kg per kg of ethylbenzene. However, heating the primary reactor to a temperature required for efficient conversion of the ethylbenzene remains a separate problem.
Additionally, International Application No.: PCT/US2013/032244, Pub. No. W0/2014/142994, relates to efficiencies in the production of styrene through reduced quantities of steam used in the disclosed process. However, there still remains a need in the art for improvements that can provide even greater efficiencies through lower heating steam to ethylbenzene ratio, as presented herein. Without a means of supplying heat to the primary reactor feed prior to the addition of superheated steam, the temperature of the superheated steam added to the reactor feed upstream of the first reactor would exceed the mechanical temperature limits of the steam transfer line and the mixing device. To bring the temperature down, the amount of reaction steam has to be increased, which increases the overall energy demand of the process.
Thus, for economic reasons and process efficiencies, it is desirable to lower the reaction steam to hydrocarbon ratio of the process due to the costs incurred in generating and superheating steam. The inventive methods and systems disclosed herein provide for a reduction of reaction steam/EB ratio while practicing azeotropic heat recovery without resorting to the use of expensive alloys.